Circle of Magic: #1 Magic in the Weaving by Tamora Pierce
The first book in a brilliant fantasy quartet. Feisty Sandry has been orphaned by the plague, and sent to a convent suitable for a girl of her noble birth. She hates it. Her strange magical powers are breaking out all over the place, but it takes a great mage to spot that she needs training. And Sandry is not the only child mage around the Circle Sea. Four massively talented misfits are soon gathered at the Winding Circle temple - and together their powers are incredible.